1. Field of the Invention
This invention relates to a method of detecting the defective portion of sealing of a moisture-tight container or package intended to preserve the contents free from moisture.
2. Description of the Prior Art
There are many kinds of moisture-tight containers or packages housing the contents in a moisture-free condition including, for example, a bag of a synthetic resin film or aluminum foil, a bag of a sheet prepared by laminating a synthetic resin film and/or a metal foil on a paper base, and a rigid container of synthetic resin such as tapper. These moisture-tight containers or packages are selectively used depending on the properties of the contents.
In general, a majority of these containers or packages housing the contents are prepared on a mass production basis by using, for example, a rapid wrapping machine and various kinds of such containers or packages are available on the market. The sealing of such containers is seemingly perfect, but moisture absorption troubles occurring within the containers or packages are not negligible. Particularly, the container or package produced by utilizing a rapid wrapping machine tends to give rise to the moisture absorption trouble.
The moisture absorption trouble is caused in many cases by defective sealing or damage done to the sealed portion in the wrapping stage, though the presence of pin holes in the wrapping material (moisture-tight sheet) or the moisture permeability of the wrapping material is sometimes responsible for the moisture absorption trouble. The defective sealing is caused by, for example, the entry of the contents in the sealed portion, the occurrence of foldings near the sealed portion or, in the case of heat seal, the split by over-heating along the inner edge of the sealed portion. In any case, the defective portion of sealing is so small that it is difficult to detect it and, thus, the defective portion tends to be overlooked. Consequently, the packed material absorbs moisture present in the air during preservation and, thus, is deteriorated. Further, where the packed material comprises a substance capable of absorbing a large amount of moisture, even an appreciable amount of moisture absorbed fails to enable the entire packed material to present a noticeable appearance. In such a case, it is difficult to recognize the defective portion of sealing.
Conventional methods for detecting the defective sealing include, for example, an optical method and a physical or chemical method. The former utilizes the irradiation of X-rays or ultraviolet rays or transmission of light so as to detect the defective portion of sealing. On the other hand, the latter utilizes a gas injection into a moisture-tight container or immersion of such a container into liquor. In this case, leakage of gas outside the container or dissolution of the packed material into the liquor is detected and measured physically or chemically.
However, the optical method necessitates a costly apparatus and involves troublesome operations of the apparatus. In addition, the result of the examination does not always reflect accurately the actual state of moisture absorption of the packed material. Suppose the optical method is applied to a bag of a laminated sheet consisting of an aluminum foil and a thermoplastic film for detecting the defective portion of sealing. In this case, even a slight damage to the aluminum foil may be detected, but the detection does not necessarily indicate moisture absorption of the packed material because moisture does not permeate through the laminate if no damage is done to that portion of the thermoplastic film which corresponds to the damaged portion of the aluminum foil. On the other hand, the physical or chemical method is not satisfactory in that it fails to locate specifically the defective portion of sealing.
An additional prior art to be noted uses an environmental test apparatus set under predetermined conditions. In this case, the defective sealing is detected by measuring the change in weight of the test piece or the desiccant housed in the moisture-tight container. Indeed, the test result of this method accurately reflects the actual state of moisture absorption of the packed material, but this method requires a considerably long testing time and fails to locate specifically the defective portion of sealing.